Traditionally, general purpose amplifiers based on the Darlington pair feedback topology exhibit a large temperature variation of the total current drawn from the power supply. The base-emitter turn-on voltage of a GaAs heterojunction bipolar transistor (“HBT”) has a negative temperature coefficient, i.e., as temperature decreases the required switching voltage increases, and vice versa. Thus, if the base or bias voltage is maintained constant while temperature varies (which occurs in conventional designs), then the current drawn by the amplifier will vary with the temperature variation. At higher temperatures the total current draw increases, while at lower temperatures the total current draw decreases. This phenomenon results in a variation of the total current draw, amplifier gain, power, and linearity. Indeed, at very low temperatures the total current draw might decrease to a point where the transistor devices turn off.
A constant bias point over temperature is desired to ensure performance parameters such as gain, power, and linearity are not degraded when the amplifier device is operated at temperatures that vary from room temperature. Traditionally, temperature variation in HBT Darlington pair amplifier circuits is controlled by the use of on-chip collector resistors in the radio frequency (“RF”) output path and/or off-chip collector resistors in the DC power supply path. The former requires the use of a larger active transistor area to account for the output power lost in the output path resistors (larger active area also requires higher current requirements). The latter solution requires operation from a higher voltage supply and decreases efficiency due to the power dissipated in the resistors. Both of these conventional solutions may be undesirable in practical applications having a limited power supply voltage and/or strict RF output power requirements.
Accordingly, it is desirable to have an on-chip temperature compensation circuit for use with a general purpose amplifier circuit, such as a GaAs HBT Darlington pair amplifier circuit. In addition, it is desirable to have an on-chip temperature compensation circuit that takes advantage of existing semiconductor fabrication processes. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.